1. Field of the Invention
The present invention relates to an alarm signal generation device for a vehicle which detects an intruder on the interior of a vehicle by use of a supersonic sensor to give an alarm.
2. Related Art
In recent years, as an intruder alarm device for prevention of crimes in a vehicle, there has been put to practical use an alarm signal generation device which uses a supersonic sensor. That is, in such alarm signal generation device, there is provided in the vehicle interior a supersonic sensor which includes a supersonic generator for radiating supersonic waves and a supersonic receiver for receiving the supersonic waves reflected in the vehicle interior, and an alarm signal is generated by use of the Doppler effect of the reflected supersonic waves responsive to the movements of the intruder. In particular, while the intruder is moving, the Doppler effect produces a difference between the respective frequencies of the radiated and reflected supersonic waves and, for this reason, a beat frequency signal corresponding to the above frequency difference is included in the signal that is received by the supersonic receiver. Thus, such frequency signal is integrated by an integrating circuit to thereby generate a voltage signal and, when the voltage signal arrives at or goes beyond a given reference level, then the alarm signal is generated.
In the above-mentioned conventional alarm signal generation device, the supersonic sensor is driven successively, which consumes a relatively large amount of current. Therefore, in a condition in which a vehicle is to be parked continuously for a long time, there is a possibility of so called run-down of a battery. To cope with such problem, for example, it can be expected to employ a structure that the supersonic sensor is driven intermittently. However, since the supersonic sensor has a nature that the output frequency thereof is unstabilized in the early start time of driving thereof, if there is simply employed the structure to drive the supersonic sensor intermittently, then there arises a problem that the detection operation of the supersonic sensor cannot be stabilized.